The present invention relates to a thread separator including a plate having a hook-shaped recess for seizing and separating threads held in an array.
Such separators are parts of a seam weaving machine for making endless sheet forming fabrics for use in papermaking machines. The array of threads is a magazine-type thread holding band in which the longitudinal threads of the sheet forming fabric are held by interweaving with transverse threads. In order to form the woven seam the longitudinal threads must be taken or separated successively one by one out of the array of threads. The longitudinal threads of such sheet forming fabrics generally have diameters within the range of about 0.15 to 0.22 mm.
The plate has a leading edge facing the array. The hook-shaped recess forms a base edge recessed from the leading edge where the thread to be separated lies before being seized and held by the hook-shaped recess upon shifting of the plate. The separator includes a frame in which at least one plate is slidably guided for seizing the foremost thread of an array. Drive means are provided for moving the plate.
Plates and separators of this design are known from DE-U-81 22 450, DE-U-87 06 649, DE-A 37 12 169, DE-A-37 14 517, EP-A-289 640 and U.S. Pat. No. 4,866,821 with each plate being formed with one or two hooks conforming to the diameter of the thread to be seized. The recess is semicircular so that the thread bears against the hook-shaped recess with half its circumference. The relation of the hook-shaped recess to the thread diameter must be very accurate in order to accomplish, on the one hand, safe gripping of the foremost thread of the array and, on the other hand, to prevent piercing and entraining of the next following thread. Nevertheless, a certain number of failures is inevitable. Failure may be due to the fact that the foremost thread is not seized or that not only the foremost but also the next following thread is seized. Each failure results in the standstill of the seaming machine.
In the separators disclosed in DE-U-81 22, 450, DE-U-87 06 649, DE-A-37 12 169, De-A-37 14 517 and EP-A-289 640, each successive thread is seized by being urged by the hook against the frame. The thread to be seized with the separators of U.S. Pat. No. 4,866 821 is grasped between the confronting hooks of at least two plates with one hook being adapted to the diameter of the thread to be separated and the cooperating other hook being of about half the size. The smaller hook pierces the thread at about half its width and urges it into the larger hook. With plates provided with hooks the positive engagement can extend maximally to half the circumference. Threads that are firmly engaged in the serial arrangement cannot therefore be safely separated.
Plates are also known where the hook is replaced by a piercing needle which enters the thread in order to grip it as in Applicants copending U.S. application Ser. No., 07/495,963 filed Mar. 20, 1990, now U.S. Pat. No. 5,003,675. However, there is a risk that the needle will extend through the thread and penetrate other threads.